The relationship between flow rate and pressure drop through a flow restriction has been employed to control flow rate. E.g., in Kates U.S. Pat. No. 3,402,735, fluid upstream and downstream of a flow restriction is exposed to opposite faces of a piston that is spring-biased in the direction of the upstream face and moves toward closing an outlet when displaced in the opposite direction owing to a temporary increase in the difference in pressure between the upstream face and the downstream face caused by increased flow.